1. Field of the Invention
The present invention relates generally to signal transmission, and more particularly, to an envelope detection device and communication device that detect transmission state and disconnect state of the signal transmission by sharing single comparison circuit.
2. Description of the Prior Art
For a communication device performing data transmission on the buses, it is important to correctly recognize transmission state of signals on the buses or connection states with other communication devices on the buses. For example, the communication device may detect signal levels on the buses by utilizing some envelope detectors. When signal levels are lower than certain thresholds, the communication device would recognize that no signals are currently on the buses. At this time, the communication device may enter specific states and may not process the signals received from the buses. Furthermore, the communication device may recognize that other remote communication device on the buses is disconnected if the signal level on the buses is, for example, higher than certain thresholds.
Taking a universal serial bus (USB) device on an USB bus as an example, the USB device may have to detect squelch state, Chirp J/K state (which is state related to handshake for switching from a reset period into a high-speed mode), and disconnect state that is related to connections between devices, depending on its operation mode (e.g. host mode or device). The receiver of the USB device may include multiple envelope detection devices (envelope detectors). These detectors are respectively responsible for comparing a potential difference between the signal lines D+ and D− of the USB bus (i.e., a USB differential signal) with multiple reference levels. For example, if it is detected that the differential signal is smaller than or equal to 100 mV, it is recognized as at the squelch state; and if it is detected that the differential signal is greater than or equal to 150 mV, it must not be recognized as the squelch state. Similarly, when the differential signal is greater than or equal to 300 mV, it is recognized as at the Chirp J state. When the differential signal is smaller than or equal to −300 mV, it is recognized as at the Chirp K state. Furthermore, when the differential signal is greater than or equal to 625 mV, it is recognized as at the disconnect state. When the differential signal is smaller than or equal to 525 mV, it must not be recognized as disconnect state.
From the above descriptions, the receiver needs multiple envelope detection devices and each envelope detection device needs respective comparison circuit and reference signal generation circuit in order to compare the differential signal on the signal lines D+ and D− with the reference signal at different levels (e.g. 100 mV, 150 mV, ±300 mV, 525 mV and 625 mV). However, the quantity of comparison circuits and reference signal generation circuits directly leads to an increase in circuitry complexity and manufacturing costs of the receiver.